Definition
A ground-based system of lights placed beside the runway that gives the pilot a visual indication of whether the aircraft is on, above, or below the correct approach path to the landing threshold. A standard VASI uses two bars of lights that appear red or white depending on the aircraft's position relative to the glide path: white over white means too high, red over red means too low, and red over white means on the correct slope.
Plain English
A set of lights next to the runway that tell the pilot, by changing color, whether the approach is too high, too low, or just right.
Context Anchor
Seen during the final part of a landing approach, especially when runway shape, slope, or surrounding terrain could make the pilot misjudge height or distance.
Derivation
Visual comes from the Latin videre, meaning “to see.” Indicator comes from Latin words meaning “to point out.” Together, the term means a system you can see that points out the correct approach slope.
Why Pilots Care
Provides an immediate visual reference that helps prevent landing short, long, or hard due to optical illusions during the approach.
Grounding Statement
If the runway picture in the windshield feels misleading, the Visual Approach Slope Indicator gives a separate light-based cue for the correct descent path.
Intuition Check
Do not read “slope” here as the physical tilt of the runway. Here, “slope” means the recommended downward path the airplane should follow toward the runway.
Example Sentence 1
On final approach the pilot saw red over white on the VASI and held the descent steady to stay on the glide path.
Example Sentence 2
During the approach the Visual Approach Slope Indicator showed all white, so the pilot lowered the nose slightly to correct the glide path.